Many heat-transfer fluids are known and used in various applications. Air and water are commonly used owing to their natural availability. Other types of known heat-transfer fluids are organic fluids, molten salts and liquid metals.
When choosing a heat-transfer fluid for a particular application, several criteria should be taken into consideration:
Absorptivity of the fluid.
Temperature range over which it will be used: maximum and minimum temperatures.
Persistence of effectiveness over time: aggressiveness of the fluid with respect to the materials used for the installation, risks of corrosion or of fouling, etc.
Safety regulations: risks of burns, electrical risks, risks of fire, risks of explosion, toxic risks, risks associated with pressurized equipment, regulations for disposal and recycling of the fluid.
Technical-economic criteria: investment and running costs.
In the case of a domestic solar collector intended to heat the domestic water of a dwelling, the heat-transfer fluid generally used is a mixture of water and antifreeze. This is because this fluid satisfactorily meets all the abovementioned criteria for this type of application.
However, this fluid has a limited absorptivity and the solar collector must therefore be designed so as to promote the conversion of light to heat. The yield from the solar collector is limited.
U.S. Pat. No. 4,083,490 describes a collector intended to absorb solar radiation, comprising an absorbent medium constituted of ethylene glycol, of water and of a colloidal suspension of graphite having particles of approximately 1 μm. The colloidal suspension of graphite behaves as finely divided black bodies which directly absorb a large amount of radiative energy.